If she's disregarding your feelings, and not trying to help you with this, that's a problem. But I agree with the other posters that it's not fair to ask her not to see her other partner. My impression is that your experience -- being so strongly connected to her that your primary may as well not be there after you see her -- is very rare.
I can tell you, as someone who's dated a married woman for two years and who's only recently had a primary of my own (and not a live-in one for the time being), there have been many times when I've left my gf's house and felt lonely as soon as I started driving away, not still magically connected, as nice as that sounds. Not to say that I leave feeling sad and lonely every time, far from it, but it can be hard, when I feel like I wanted more time but had to let her go anyway.
Sometimes after seeing her, I go home and sleep alone. Sometimes I go to my bf's place and spend the night with him. I base that decision on how he and I are doing and whether I want to see him, not on anything between her and I. I'll see her again when I next see her and that's that.
If she tried to tell me not to ever see him right after seeing her, I would be hurt, frankly, because that would mean I'd be forced to be doubly alone... not with her, and barred from seeing him. Sometimes, like I said, I choose that for myself. But if I'm lonely from missing her, and had to also deny myself his company if I wanted it and he was available, I think that would just magnify any sense of isolation I was already feeling to the point of intolerability, plus it might well negatively affect my relationship with him because we'd be missing chances to connect, and time can be such a rare and previous commodity in the lives of poly folk.
So, I guess what I'm saying is I'm sorry you're hurting, and your gf should be sensitive to that, but I sympathize with her 1,000% on this one.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.